Hi Sarah
Yes, you can do this in Admin - simply create a new calculated field and setup the usual items for alias etc. Then, in the calculation make sure the result type is numeric. Then, in the edit box for the calculation type into the area what you need - which will be something like
DateDiff(d,GetDate(),mySQLColumn)
This will then be run within SLX and SQL Server will do the calculation for you.
However, you will probably have to have a little customisation done so that it stores what they picked (30/60/90) and can then calculate the difference (instead of Today you would use:
DateDiff(d,MyCol1, myCol2)
The result being the difference of the two. In myCol1 would be the offset (30/60/90 etc).
Regards
Mike